A variety of marker detection systems such as EAS and RFID systems are available to protect and track assets. In an EAS system, an interrogation zone may be established at the perimeter, e.g. at an exit area, of a protected area such as a retail store. The interrogation zone is established by an antenna system positioned adjacent to the interrogation zone. The antenna system may include one or more antennas to establish an electromagnetic field within the interrogation zone.
An EAS marker is attached to each asset to be protected. When an article is properly purchased or otherwise authorized for removal from the protected area, the EAS marker is either removed or deactivated. If the marker is not removed or deactivated, the field causes a response from the EAS marker in the interrogation zone. An antenna acting as a receiver detects the EAS marker's response indicating an active marker is in the interrogation zone. An associated controller provides an indication of this condition, e.g., an audio alarm, such that appropriate action can be taken to prevent unauthorized removal of the item.
An RFID system utilizes an RFID marker to track articles for various purposes such as inventory. The RFID marker stores data associated with the article. An RFID reader may scan for RFID markers by transmitting an interrogation signal at a known frequency. RFID markers may respond to the interrogation signal with a response signal containing, for example, data associated with the article or an RFID marker ID. The RFID reader detects the response signal and decodes the data or the RFID marker ID. The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID marker pass. A fixed reader may be configured as an antenna located in a pedestal similar to an EAS system.
Many markers for use in such marker detection systems have a single favored orientation with respect to the stimulating field where they exhibit a maximum response, i.e., they are directional. Most markers are somewhat rectangular in shape with a high length-to-width ratio. These markers give a maximum response when oriented within a field such that the field flux coincides with the long axis of the marker. These markers tend to have little or no response when the field flux lines are substantially orthogonal to the long axis of the marker. In this instance, a vector component of the electromagnetic field in the interrogation zone in the same direction of the long axis of the marker is not sufficiently strong to provide for reliable marker detection.
Such areas of a weak electromagnetic field component in one direction within certain regions of the interrogation zone are referred to herein as “null zones.” Such null zones degrade the performance of the marker detection system as a marker passing through a null zone in a certain orientation may not be properly detected. Therefore, it is desirable for marker detection systems to have a sufficiently strong and uniform electromagnetic field in many orientations across the plane of the interrogation zone in order to provide for reliable marker detection.
Various antennas, including loop and magnetic core antennas, have a drawback in that they exhibit at least one significant null zone (whether used for transmitting or receiving) for some particular orientation of the marker. For example, with loop antennas these null zones are related to the loops' axes of symmetry.
Because of this tendency to form null zones, recent marker detection antenna systems typically contain a plurality of antenna elements to allow operation either out-of-phase (field canceling) or in-phase (field aiding). However, such field canceling and field aiding elements are not simultaneously driven. Field canceling arrangements are designed to establish a strong field in the interrogation zone and a diminished field far away from the antenna to comply with regulatory requirements. Such regulatory limits specify maximum field readings at proscribed distances from the antenna system beyond the interrogation zone.
Such a field canceling arrangement for a loop antenna may include a nested loop configuration where an inner loop antenna is nested within an outer loop antenna in a common plane. The outer loop antenna and inner loop antenna are designed so that at least a portion of the electromagnetic fields from each of the loops are equal and opposite at a distance far away from the antenna causing such fields to cancel. Field aiding arrangements are designed so that two or more smaller antennas may produce fields in similar directions so that at least a portion of the electromagnetic fields add together.
However, even in such systems utilizing multiple antennas, there may be certain regions where the electromagnetic field vectors from adjacent antennas cancel one another to contribute to the formation of null zones. In addition, for wire loop antennas the wire loop behaves like an inductor that may be resonated or tuned by selecting an appropriate value of a resonating capacitor. Operating these multiple elements both in phase and out-of-phase usually requires different tuning adjustments when the mutual inductance between the multiple coils changes.
Accordingly, there is a need for an antenna system that establishes a marker interrogation field with minimal null zones in a facile and efficient manner.